PLED devices containing triphenylamine-derived polyurethanes as hole-transporting layers exhibit high current efficiencies

Abstract

The innovative polyurethane-type polymers from a triphenylamine derivative, [N,N′-bis(4-hydroxyphenyl)-N,N′-diphenylbenzidine] (TPA) and a carbazole derivative, [9-butyl-3,6-bis(4-hydroxyphenyl)carbazole] (Cz) can be linked through isophorone diisocyanate (IPDI) bridges and incorporated as hole-transporting layers in high-performance PLED devices. The TPA–IPDI–Cz type of polyurethane (PU) materials (P1–P5) showed superb hole injection and transport properties based on the results of the hole-only-device study. We prepared the devices in two kinds of configuration: system (1) Indium Tin Oxide (ITO)/PU (20 nm)/{Iridium(III) bis(2-phenylpyridine), [Ir(ppy)₃] + 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (t-PBD) + polyvinylcarbazole (PVK)] (50 nm)/Mg (10 nm)/Ag (100 nm), for the device with P1 (DP1), the brightness increased to 14 000 cd m⁻², the current efficiency rose to 13.4 cd A⁻¹, and the turn-on voltage was reduced to 21 V (at 100 cd m⁻²). In system (2) ITO/PEDOT–PSS (30 nm)/PU (20 nm)/[Ir(ppy)₃ + t-PBD + PVK] (50 nm)/Mg (10 nm)/Ag (100 nm), with PEDOT–PSS as the hole-injection layer, was compared to the standard device (S2) having the configuration ITO/PEDOT–PSS (50 nm)/[Ir(ppy)₃ + t-PBD + PVK] (50 nm)/Mg (10 nm)/Ag (100 nm); the brightness of the double layer device with P5 (DDP5) increased to 12 500 cd m⁻² and the current efficiency dramatically rose to 34.7 cd A⁻¹, compared with values of 6250 cd m⁻² and 21.8 cd A⁻¹, respectively, for S2.